[Answered ]-Django passing data through the url, not finding page in url patterns

It should be view_appointment/{{a.id}}, not adminview_appointment/{{a.id}}, but it is better to make use of the {% url … %} template tag ^[Django-doc]. You can give the view a name:

path('view_appointment/<int:id>/', viewAppointment_view, name='view_appointment'),

and then refer to it in the template:

<a href="{% url 'view_appointment' id=a.id %}">Client name:{{a.client_dog_name}}</a><br>